The invention relates to a method of growing mixed crystals having at least two lattice sites with each a different number of adjacent oxygen ions from melts of oxidic multi-component systems and also to the use of the mixed crystals produced in accordance with this method.
In order to obtain homogeneous crystals, the requirement must be satisfied that the distribution coefficient of the cations k.sub.eff(kat), in such that the ratio of the concentration of the cations in the solidus phase to the concentration of the cations in the liquidus phase C.sub.s /C.sub.1 is at approximately 1. Crystals suitable for use in technical applications are obtained at low cation concentrations (.ltoreq.0.5 formula units) still with distribution coefficients in the range from 0.90 to 1.20.
Crystals must here be understood to include both monocrystalline and polycrystalline material.
Garnet single crystals are, for example employed as substrates for the production of magnetic and magneto-optical layers. Examples in which such layers of materials are used are: magnetic bubble memories, displays and optical isolators. In addition, garnet single crystals can also be used as substrates for the epitaxial production of semiconductor layers.
A further field of application for garnets are laser crystals and detectors. In these examples garnets are utilized as host lattices for fluorescent ions. In addition to the production of compact laser crystals, garnet single crystals are also employed as substrates for the production of thin layers with laser properties.
Single-crystal layers can be deposited by, for example an epitaxial process, for example a liquid-phase epitaxial process, onto single-crystal substrates. A single-crystal growth of the layer with the necessary perfection can however only be effected when the substrate and the epitaxial layer have substantially the same crystallographical lattice constant. The lattice constant changes with respect to composition, that for each layer composition a substrate with a matched lattice constant is required.
By way of example, desired lattice constants for garnet substrates for the aforementioned applications are located within the range from 1.19 to 1.30 nm. This lattice constant range can not be covered entirely with the garnet compositions which have been known so far.
A solution therefore is the growth of mixed crystals in which the lattice constants across is adjusted the crystal composition. This does not only hold for garnets but in general for growing single-crystal layers on monocrystalline substrates. For technical uses also perowskites and spinels are of increasing significance.
These problems will be described in greater detail with reference to garnet compounds.
Garnet crystallizes with a cubic crystal structure. There are three different lattice sites, (dodecahedral, octahedral and tetrahedral sites) for the cations in the garnet lattice. Cations having the largest ion radii first occupy the dodecahedral site up to 3 formula units and have as their nearest neighbours 8 oxygen ions. Cations having the next but one smallest radii occupy the octahedral site and have 6 oxygen ions as their nearest neighbours, while the smallest cations occupy the tetrahedral site and are surrounded by 4 oxygen ions only. The lattice sites are interconnected contiguous to each other via their sides and their corners. The different lattice sites are characterized by the following types of brackets: dodecahedral site { }, octahedral site [ ], tetrahedral ( ).
Garnets can be described by the general formula {A.sup.3+ }.sub.3 [B.sup.3+ ].sub.2 (C.sup.3+).sub.3 O.sub.12, wherein A, B and C are cations having different radii. In garnets of a simple composition, such as, for example {Gd}.sub.3 [Ga].sub.2 (Ga).sub.3 O.sub.12, the dodecahedral sites are occupied by Gd.sup.3+ -ions, the octahedral and tetrahedral sites by Ga.sup.3+ -ions.
A known method is to produce mixed crystals by mixing two garnets of a single composition (alternatively denoted terminal members). Thus, for example, mixed crystals of the composition {Gd,Sm}.sub.3 [Ga].sub.2 (Ga).sub.3 O.sub.12 can be produced by mixing the two terminal members {Gd}.sub.3 [Ga].sub.2 (Ga).sub.3 O.sub.12 and {Sm}.sub.3 [Ga].sub.2 (Ga).sub.3 O.sub.12. In these mixed crystals the smaller Gd.sup.3+ -ions (r=0.1061 nm) are successively exchanged for the larger Sm.sup.3+ ions (r=0.1087 nm) in the dodecahedral sites. The lattice constants of these mixed crystals increases linearly versus an increasing Sm content from 1.2382 nm up to 1.2438 nm.
Mixed crystals obtained by mixing terminal members have the disadvantage that there is a change in the concentration of the cations occupying the dodecahedral sites in the crystal from the beginning up to the crystal end; for the aforementioned examples this would be the Gd/Sm concentration.
A list for these inhomogeneities is the distribution coefficient of the cations k.sub.eff(kat). This is defined as the ratio of the cation concentration in the crystal to the cation concentration in the melt (k.sub.eff(kat) =C.sub.s /C.sub.l). In the above example the distribution coefficient of gadolinium exceeds 1. This results in an enhancement of gadolinium in the crystal beginning and a corresponding decrease of the Gd concentration in the melt during the growing process in a lower proportion of gadolinium in the crystal end.
Because of the dependence of the lattice constant on the crystal composition the lattice constant increases from the crystal beginning up to the end.
Homogeneous mixed crystals, which are mixed crystals which have an approximately equal lattice constant across the overall length of the drawn crystal, can only be obtained when the distribution coefficient of the cations is approximately 1. Mixed crystals which are produced by mixing two terminal members have however distribution coefficients which deviate relatively large from 1. Because of the resultant poor crystal quality and the change in the lattice constant in the longitudinal direction of the crystal, mixed crystals produced by mixing two terminal members are generally not suitable for use for technical applications. A solution might be the use of multi-component systems which, because of the higher number of cations, provide a greater number of possibilities to optimize the distribution coefficient across the compound. How to achieve this is however not known yet.